Music for piano four-hands, that is two pianists playing at the same piano, is often printed with the two parts on different pages. The "primo" (top) is on the right pages, and the "secondo" (bottom) is on the left pages. So the two players of that Mozart sonata would not be looking at the same place.
When Heidi and Paul are playing piano together (looking at vs. reading the sheet music), they're obviously not playing the same measure, as he's looking at the left page and she's looking at the right page. It's right before Heidi laughs and points out something, to Paul, about the way it was played before by a certain musician. It revealed the fact that the actors were not real musicians, knowing how to read music.